Naming Include Guards

Question

How are C++ include guards typically named? I tend to see this a lot:

#ifndef FOO_H
#define FOO_H

// ...

#endif

However, I don\'t think

Answer 1

As others mentioned before, a very common convention is to use the uppercase version of the name, and the dot replaced by an underscore: foo.h -> FOO_H

However, this can lead to name collisions with simple and/or common names. For this reason, autogenerated header like the stdafx.h in non-empty Visual C C++ projects append some random string, like:

#ifndef FOO_H__NsknZfLkajnTFBpHIhKS
#define FOO_H__NsknZfLkajnTFBpHIhKS
#endif

http://www.random.org/strings/ is a useful random generator for this.

Also, if the file is part of some submodule, or its contents reside in one specific namespace, I tend to add that to the guard too:

#ifndef SOMECOMPONENT_FOO_H__NsknZfLkajnTFBpHIhKS
#define SOMECOMPONENT_FOO_H__NsknZfLkajnTFBpHIhKS

namespace somecomponent
{
  ...
}

#endif

Answer 2

I normally use something like FOO_H_INCLUDED_. A few (Microsoft) headers have what looks a lot like a string representation of a GUID, but I've never needed anything quite that elaborate.

Answer 3

I personally follow Boost's recommendation. It's perhaps one of the largest collection of C++ libraries of good quality around and they don't have problem.

It goes like:

<project>_<path_part1>_..._<path_partN>_<file>_<extension>_INCLUDED

// include/pet/project/file.hpp
#ifndef PET_PROJECT_FILE_HPP_INCLUDED

which is:

• legal (note that beginning by _[A-Z] or containing __ is not)
• easy to generate
• guaranteed to be unique (as a include guard) within a project (else you have two files at the same place)
• guaranteed not to be used for anything else (if you end another macro with INCLUDED you're spoiling for a fight)

I've read about GUID but those look weird.

And obviously I'd rather than all compilers implement #pragma once (or better, #pragma multiple and "once" be the default behavior...)